Efficient diversity combining for wideband downlink

ABSTRACT

A communication system employing site diversity combing to increase link availability includes at least two receivers at receive sites within a single downlink beam separated by enough distance to provide decorrelation of weather phenomena—such as rain fade outages. A signal transmits digital symbols to all the receivers and may use bandwidth efficient modulation with forward error correction coding. Sampled symbol values for each codeword are produced at each receiver, which are connected by one or more ground links so that all data can be collected at one site. At least two different soft-decision computation modules translate the sampled symbol values from the different receivers into different sets of soft-decision values—which may be log-likelihood-ratio (LLR) values reflecting the probability value for each bit of the codeword—that are digitally synchronized and combined for use by a decoder. The technique thus avoids disadvantages of either coherent waveform combining or BER-based digital switching.

This application is a continuation of application Ser. No. 11/165,915,filed Jun. 24, 2005, status allowed.

BACKGROUND OF THE INVENTION

The present invention generally relates to radio frequency communicationsystems and, more particularly, to a site diversity technique forproviding high link availability on a satellite or other radio frequency(RF) link that is challenged by thin power margins and high probabilityof deep rain fade.

Site diversity refers generally to a set of techniques used to providehigh link availability on a satellite or other RF link that ischallenged by thin power margins and high probability of deep rainfades, such as a wideband bandwidth efficient modulation (BEM) link. Itis not always possible to locate a BEM receive station in a lowprecipitation area of the world. In some cases, it may not be possibleto build a link with ample power margin to overcome all high or moderateprobability rain fades. Rather than over-sizing the link to carry largerain-fade power margins, which could be cost-prohibitive, site diversitymay be desirable to lower the required power margin and increase linkavailability. Using site diversity, the communication system receivesthe transmitted signal into two or more geographically separated receivelocations, generally within the same downlink beam. These receivelocations are separated far enough in distance to provide decorrelationof atmospheric phenomena so that; for example, if it is raining hard onone receiver (producing a deep rain fade), it is probably not raining ashard (thus lower rain fade) over another receiver. Thus, by suitablycombining information from both or all received signals it is generallypossible to reconstruct the original information from the transmittedsignal with a greater degree of accuracy than would be possible usingonly one signal. Thus, link availability, e.g., the percentage of timethat a link can provide a signal of acceptable quality for accuratereconstruction of the original information, can be increased incomparison to a link that uses only one receiver. But previous methodsof combining multiple received signals are usually impractical or verycostly for a very high data rate BEM link.

For example, prior art techniques for site diversity include one thatinvolves coherently combining the received RF or intermediate frequency(IF) waveforms (referred to as “coherent waveform combining”) andanother that involves demodulating the received data streams and making“either-or” decisions down stream about which packets to accept fromwhich stream (referred to as “digital switching”).

To perform the first technique, coherent waveform combining—assuming forthe sake of example, two receive stations—some component of thecommunication system must perfectly align and then combine both RFsignals at one common location with negligible distortion. Such perfectalignment is not feasible for ultra-wideband communications, especiallywith BEM formats that are highly distortion-sensitive.

The other technique, digital “either-or” switching, requires very fastdecision-making capabilities, which can be either not feasible or costprohibitive for the high frequencies and data rates involved. Moreover,the process of decoding the signals involves at one stage making a “softdecision” about the data symbol received and then refining the softdecision into a “hard decision” at a later stage of the decodingprocess. Digital “either-or” switching requires very fastdecision-making capabilities, and does not provide optimal performance,since decoding must be performed on the individual data streams, leavingonly the information from the “best” stream to be kept, and anyadditional possible performance benefit of the other stream is wasted.

As can be seen, there is a need for optimally combining two or morereceived signals to obtain a single received data stream without thedifficulties of RF coherent combining and without the performance lossof digital switching methods. There is also a need for an efficient, lowcost, low complexity, and high performance method of combining two BEMdownlinks separated by significant distance that achieves optimalrain-fade resistance. Moreover, there is a need for BEM signal combiningin which not only is the unit cost low, but the total systemrequirements, and therefore the total system cost, is also very low.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a communication systemincludes at least two receivers that receive a transmitted signal. Oneof the receivers produces a first stream of sampled symbol values, and asecond receiver produces a second stream of sampled symbol values. Afirst computation module translates the first stream of sampled symbolvalues into a stream of first soft decision values (using, for example,log likelihood ratio (LLR)) and a second computation module translatesthe second stream of sampled symbol values into a stream of secondsoft-decision values. A combining module combines the firstsoft-decision values and the second soft-decision values.

In another embodiment of the present invention, a receive site for asite diversity-combining communication system includes: a firstcomputation module that translates sampled symbols from the transmittedsignal into a stream of first soft-decision values from the transmittedsignal. A connection to at least one other site provides data for astream of second soft-decision values from the transmitted signal. Acombining module combines the first soft-decision values and the secondsoft-decision values into combined soft-decision values for each symbolof the transmitted signal.

In still another embodiment of the present invention, a sitediversity-combining communication system includes: first and secondreceive sites connected to a ground link. A first receiver located atthe first receive site receives a modulated carrier signal transmittinga coded stream of symbols. A second receiver located at the secondreceive site also receives the modulated carrier signal transmitting thesame coded symbols. A first demodulator produces a first steam ofsampled symbol values from the coded symbol stream. A second demodulatorproduces a second stream of sampled symbol values from the same streamof coded symbols. A first soft-decision computation module translatesthe first sampled symbol values into a first set of soft-decision values(which may be, for example, LLR values) for the stream of coded symbols.A second soft-decision computation module translates the second sampledsymbol values into a second set of soft-decision values for the streamof coded symbols. A combining module combines the first set ofsoft-decision values and the second set of soft-decision values toproduce a set of combined soft-decision values for the stream of codedsymbols. Data is sent from the first and second receive sites to thecombining module via the ground link so that the first and second setsof soft-decision values are provided to the combining module.

In yet another embodiment of the present invention, a communicationsystem using site diversity combining for high link availabilityincludes: first and second receive sites connected to a transmitter viaa combined link that transmits a stream of coded symbols using amodulated carrier signal. The first and second receive sites areseparated by a sufficient distance to provide decorrelation of weatherphenomena. The first receive site includes a first receiver thatreceives the modulated carrier signal and a first demodulator thatdemodulates the carrier and produces a first sampled symbol value fromthe coded symbol. The second receive site includes a second receiverthat receives the modulated carrier signal and a second demodulator thatdemodulates the carrier and produces a second sampled symbol value fromthe same coded symbol. The first and second receive sites are connectedvia a ground link so that a joint soft-decision computation modulereceives the first sampled symbol value and the second sampled symbolvalue. The joint soft decision computation module then computes thejoint soft-decision output value for the stream of coded symbols. Adecoder connected to the combining module produces a set of decoded bitvalues for the coded symbol.

In a further embodiment of the present invention, a satellitecommunication system includes a transmitter that is located on thesatellite and that transmits coded symbols via a modulated carriersignal, using a bit mapping of coded symbols to a modulation scheme,over a combined link comprising at least two links, i.e., a first andsecond link. The first link connects a first receive site to thetransmitter, the first receive site including a first receiver thatreceives the modulated carrier signal and a first demodulator thatdemodulates the carrier and (referring now to one coded symbol inparticular) produces a first sampled symbol value I,Q pair from thecoded symbol. The second link connects a second receive site to thetransmitter, the second receive site including a second receiver thatreceives the modulated carrier signal and a second demodulator thatdemodulates the carrier and produces a second sampled symbol value I,Qpair from the same coded symbol. The first receive site and the secondreceive site are separated from each other by a sufficient distance toprovide decorrelation of weather phenomena. The first receive site andthe second receive site are connected via a ground link to provide thefirst and second sampled symbol value I,Q pair as two coded input symbolstreams to a joint soft-decision computation module. The jointsoft-decision computation module then computes the joint soft-decisionoutput value for the two coded input symbol streams. A decoder connectedto the combining module produces a set of decoded bit values for thecoded symbol.

In a still further embodiment of the present invention, a method forsite diversity combining, includes the steps of: receiving a signal atat least two receive sites; translating the signal from each of thereceive sites into soft-decision values; and combining the soft-decisionvalues at one of the receive sites, the combined soft-decision values tobe used by a decoder.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a satellite communication system in accordancewith one embodiment of the present invention;

FIGS. 2A, 2B, and 2C are system block diagrams showing alternativeexemplary embodiments of a downlink, receiving subsystem, in accordancewith possible embodiments of the present invention, for a communicationsystem such as that shown in FIG. 1;

FIG. 3 is phase plane diagram for a 16-QAM modulation scheme used as anexample to illustrate one embodiment of the present invention;

FIG. 4 is a graph illustrating performance comparison between diversitycombining in accordance with an embodiment of the present invention andbit error rate based diversity combining; and

FIG. 5 is a flowchart of a method for diversity combining in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention, since the scope of theinvention is best defined by the appended claims.

Broadly, the present invention provides site diversity combining toovercome atmosphere induced link outages and increase communicationsystem link availability, in particular, for ultra-wideband bandwidthefficient modulation (BEM) links that may otherwise not be able tooperate according to required link availabilities. One embodimentenables a very simple and low cost solution to the site diversitycombining problem, enabling use of wideband BEM downlinks into regionswith high rain fade characteristics. For example, one embodiment canutilize components already present in a wideband BEM link and BEMreceiver chain, with some simple modifications and a high-speed digitalconnection between receive sites. One embodiment can perform optimalsoft-decision combining on two or more BEM waveforms originated from asingle transmitter and received into sites geographically separated byan arbitrary distance but within the same receive beam. The distance maybe nominally 10.0 kilometers (km), e.g., between 3 km and 100 km—thegeographic separation being required to provide decorrelation ofatmospheric phenomena—such as rain cells—and provide increased linkavailability.

A novel feature of one embodiment of the present invention is the use ofcombining at soft-decision outputs (for example, log-likelihood-ratio(LLR) table outputs) to provide optimal performance combining with lowcomplexity. One embodiment of the present invention, by using sampledsymbol values from different receivers, achieves a performance benefitsimilar to that of prior art optimal coherent combining techniques overdigital “either-or” switching based schemes that don't perform verywell, but without requiring the difficult-to-achieve alignment anddistortionless re-transmission of wideband BEM radio frequency (RF)signals that would be required by those prior art techniques. Oneembodiment provides less complicated and better performing sitediversity combining than digital “either-or” switching based schemes. Asignificant feature of one embodiment is its novel solution tosynchronization problems encountered in trying to synchronize twowideband RF signals. Rather than trying to synchronize two wideband RFsignals, the novel approach uses existing framing information utilizedfor the block coding/decoding function, to achieve digitalsynchronization that may be characterized as simple, easy, and cheap bycomparison to prior art techniques of RF synchronization that aretypically analog in nature, rather than digital, and so are difficult tomaintain at the precision needed. Key points are: (1) combining forwarderror correction coding (FEC) with the coherent combining functionsignificantly reduces the challenges and complexity of diversitycombining; (2) embodiments address the processing challenges of highspeed implementation, for example, simple synchronization and minimalprecision analog to digital converters (A/D); (3) for satellite links,the performance for one embodiment is very close to the optimal achievedby joint equalization/combining.

One embodiment improves practicality by reducing the dynamic range ofthe signal sent from the receive site to the decoder. Rather thanrepresent the entire dynamic range, automatic gain control is appliedprior to transmission to the decoder. Computation of the correctsoft-decision metric (e.g., log-likelihood-ratio) is accomplished byalso transmitting the constituent downlink signal-to-noise ratio to thelog-likelihood computation. Since the signal-to-noise ratio variesslowly compared to the signal, the bandwidth required is much less thanneeded to represent the non-gain controlled signal to a similar level offidelity. This can significantly enhance the applicability ofembodiments of the present invention since the high sample rateanalog-to-digital converters are only available with limited precision.Implementation of this improvement can be further simplified byutilizing the automatic gain control setting as a surrogate for thesignal-to-noise ratio. Once the system noise level is calibrated theautomatic gain control setting provides a high fidelity estimate of thesignal-to-noise ratio.

FIG. 1 illustrates communication system 100, employing site diversitycombining in accordance with one embodiment of the present invention.Communication system 100 may include a satellite 102 having RFcommunications equipment—such as transmitters and receivers—forcommunication with ground stations 104—which may include receive site104 a and receive site 104 b. Communications may be provided over acombined link 109 by a single downlink beam 108, which may includemultiple links—such as link 109 a between satellite 102 and receive site104 a and link 109 b between satellite 102 and receive site 104 b. Forexample, links 109 a, 109 b may be BEM links that transmit a BEMmodulated RF carrier signal 103. Although communication system 100 isillustrated with two ground stations and two links, any practical numberof ground stations 104 and corresponding links 109 could be used inaccordance with alternative embodiments of the present invention. Links109 may be subject to rain fade phenomena caused by atmosphericdisturbances such as rain 110 and other factors affecting linkavailability, which may be described as the percentage of time that arain loss power allocation is not exceeded. Ground stations 104, inparticular, receive sites 104 a and 104 b may communicate over a groundlink 106. Ground link 106 may be, for example, a high-speed digitalconnection or any suitable connection for achieving synchronization ofdata between the sites 104 or site 105 (FIG. 2C). The ground link 106may be implemented, for example, using a fiber optic network, microwavelink, wire, or any other suitable communication medium. Receive sites104 a and 104 b, for example, may be separated by a distance 112,labeled “D” in FIG. 1. Distance 112, for example, may nominally be about10 km to provide decorrelation of atmospheric phenomena for sitediversity, however, the two (or more) receive sites must be within thesame downlink beam 108. For example, link 109 a may be subject to rainfade from rain 110 while weather is clear for link 109 b, so that link109 b is not subject to rain fade at the same times—i.e., at timeshighly correlated—as link 109 a. Thus, by combining data from receivesites 104 a and 104 b over ground link 106, overall link availabilityfor links 109 can be improved over the link availability for any onesingle link—such as link 109 a.

FIG. 2A illustrates a receive system 200 using site diversity combiningaccording to one embodiment. FIGS. 2B and 2C illustrate receive systems200′ and 200″ for site diversity combining according to alternativeexemplary embodiments. Receive systems 200, (i.e., 200, 200′, 200″) mayinclude a first receive site 104 a and second receive site 104 b.Receive site 104 a and receive site 104 b may be separated by a distance112 within the same downlink beam 108 and connected by a ground link106, as shown in FIG. 1.

Each receive site 104 a, 104 b may include a receiver 202, which mayinclude an antenna 204 for receiving an RF signal 205 in the form of amodulated carrier on a link 109, from a transmitter, which may belocated, for example, on satellite 102. Signal 205 may be BEM modulated,for example, using quadrature amplitude modulation (QAM), e.g., 256-QAMor 64 QAM, or any other suitable modulation format for transmitting dataas digital codeword symbols—such as quadrature phase shift keying(QPSK), binary phase shift keying (BPSK), and amplitude phase keying(APK). Data in signal 205 may be encoded using an iterative blockcode—such as a turbo product code or low density parity check code. Eachreceiver 202 may include an RF front end 206 that receives the modulatedcarrier signal 205 and prepares it for demodulator 208. Demodulator 208may demodulate the signal 205 and perform analog-to-digital conversion(ADC or A/D) sampling to provide a data stream of sampled symbol values209 of coded symbols.

As shown in FIG. 2A, one of the streams of sampled symbol values, e.g.,symbol values 209 a, then may be transmitted via fiber optic or otherdigital means over ground link 106, and both (or all) streams collectedat one receive site, e.g. receive site 104 b. The decoding process thenmay be begun on each data stream, i.e., coded symbol values 209, bysoft-decision computation modules 210, which may be implemented, forexample, as a turbo-decoder log-likelihood-ratio table-lookup function,and produce soft-decision output values 211, which may be, for example,LLR values. While LLR is one implementation used as an example toillustrate one embodiment of the present invention, another approachthat may be used is to do normal weighted coherent combining prior tothe LLR or soft decision metric computation. The synchronizationapproach illustrated by use of ground link 106 leverages the existingforward error correction (FEC) synchronization to reduce the problem ofdiversity combining to ambiguity resolution. In other words, combiningFEC coding with the coherent combining function significantly reducesthe challenges and complexity of diversity combining. Also, asoft-decision computation module 210 and a soft-decision combiningmodule 212, as shown in FIG. 2A, may be combined into a single jointsoft-decision computation module 220 as seen in FIG. 2C.

Alternatively, as shown in FIG. 2B, the soft-decision output values 211may be computed at each receive site—e.g., receive sites 104 a and 104 bin the two-receive site example being used for illustration—and one ofthe streams of soft-decision output values 211, e.g., soft-decisionoutputs 211 a, then may be transmitted via fiber optic or other digitalmeans over ground link 106, and both (or all) streams of soft-decisionoutput values 211 collected at one receive site, e.g. receive site 104b. In either case, whether the data is in the form, for example, ofsampled symbol values of coded symbols or soft-decision output values(e.g., LLR values), sufficient data may be provided via ground link 106(or multiple links for more than two sites) so that soft-decision outputvalues 211 from all receive sites are provided to soft-decisioncombining module 212.

In another alternative, as shown in FIG. 2C, all demodulators 208 may belocated at an intermediate site 105, with intermediate frequency (IF)band data 207 passed over ground links 106 to be collected at site 105.All of the different data streams, e.g., sampled values 209 of codedsymbols, may be processed by a joint soft-decision computation module220 that performs the functions of soft-decision computation modules 210and soft-decision combining module 212. As shown in the figures, thejoint soft-decision computation module 220 may be located at any of thereceive sites—such as sites 104 a, 104 b—or an intermediate site orcentral site—such as site 105—so long as a data ground link 106 connectsthe site (whether a receive site or other site) at which jointsoft-decision computation module 220 resides so that data from at leasttwo receive sites is provided to joint soft-decision computation module220.

Among the alternatives illustrated in FIGS. 2A-2C, as well as othersthat may exist, an optimal option may be chosen as one that involves thelowest data throughput, lowest cost digital link (or links)—such as link106—between receive sites—such as receive sites 104 a, 104 b. Regardlessof the alternative chosen, existing framing information in the datastreams passed within and between sites may be used to implement digitalsynchronization of the data streams. The synchronized data streams maybe, for example, a first stream of sampled symbol values 209 and asecond stream of sampled symbol values 209 a as shown in FIG. 2A. Thesynchronized streams may also be, for example, a first stream of softdecision values 211 and a second stream of soft decision values 211 a asshown in FIG. 2B. As shown in FIG. 2C, the synchronized streams may be,for example, the first and second streams of sampled symbol values 209at intermediate site 105. The framing information used, for example, maybe existing framing information utilized for the block coding/decodingfunction performed at demodulator and ADC sampling modules 208 and whichmay be made available in the different data streams. With reference toFIGS. 2A-2C, for example, digital synchronization may be performed wherethe data streams come together, for example, at soft decision combiningmodule 212 or joint soft decision computation module 220.

As shown in any of FIGS. 2A-2C, soft-decision combining module 212, orjoint computation module 220, may produce combined soft-decision values213, which may be, for example, combined LLR values such as by adding oraveraging. The combined soft-decision values 213 may be considered ascontaining the best available information about the data (coded symbols)from modulated carrier signal 205, and may pass the combinedsoft-decision values 213 to decoder 214, which may be implemented, forexample, as an iterative block decoder such as a turbo product codedecoder or low density parity check code decoder. Decoder 214 mayproduce a set of decoded bit values 215 that reconstruct the codedsymbol data originally transmitted by satellite 102 with a higher linkavailability than for any single receive site.

A simplified illustration of the functions of soft-decision computationmodule 210 and soft-decision combining module 212 is given withreference to FIG. 3. The example illustrated by FIG. 3 assumes that16-QAM is transmitted so that any 16-QAM codeword symbol 306 may have 2bits (4 possible distinct values) on the in-phase (I) axis 302 and 2bits (4 distinct values) on the quadrature (Q) axis 304 of phase plane300. Thus, there are 16 possible distinct values for the 16-QAM symbols306, each represented by a dot in FIG. 3, and each of which can convey4-bits of information for each symbol 306 transmitted. FIG. 3 shows onepossible mapping of the 4 bits to the 16-QAM symbols 306, where a stringof 4 bits, represented as “0” or “1” appears next to the dotrepresenting each symbol 306. Each string of bits may be referred to as(b0,b1,b2,b3).

A possible received sampled symbol value 209, which may be received byone of receivers 202, is represented by an ‘x’ in FIG. 3. The A/Dconversion of demodulators 208 may produce a sampled symbol value 209that has 8 bits (256 possible values) on the I-axis 302 and 8 bits (256values) on the Q-axis 304. Thus, sampled received symbol value 209 maybe located at a point on phase plane 300 not coincident with one of the16-QAM symbols 306, and located by a value I measured along I-axis 302and a value Q measured along Q-axis 304. The values I and Q (sampledsymbol values) determining the location of received symbol value 209 maybe fed into the soft-decision computation module 210, which may beimplemented, for example, as an LLR lookup table. Soft-decisioncomputation module 210, e.g., the LLR table, may translate the receivedsampled symbol value 209 (16-bits in the example, 8 sample bits for Iand 8 sample bits for Q) into an n-bit representation ranging in valuefrom 0 to 2^(n)−1. The value of n-bit representation may be arepresentation of the confidence with which each of the 4 bits(b0,b1,b2,b3) of the 16-QAM bit mapping may be assigned to a logic ‘1’or ‘0’, in which the log-likelihood ratio is used as a measure of theconfidence in assigning an individual mapping bit to be a logical ‘one’or ‘zero’ given a received symbol value 209. The soft-decision—in thisexample, LLR—output values 211 may be comprised of such n-bitrepresentations, referred to as “LLR values”. For example, for thereceived symbol value 209 represented by x in FIG. 3, having bits(b0,b1,b2,b3), each bit may receive an n-bit representation having avalue from 0 to (2^(n)−1) so that the soft-decision LLR output value 211for symbol 209 may be a set of four LLR values (v0,v1,v2,v3).

The LLR soft-decision output values 211 may be combined by soft-decisioncombining module 212 for each symbol transmitted and received at two ormore receive sites. For example, the LLR soft-decision output values 211may be combined by adding to give a combined soft-decision LLR value(v0,v1,v2,v3) 213 for the particular symbol, and the value (v0,v1,v2,v3)213 may be passed to decoder 214 which may be designed to work withvalues over an appropriate range of values, as understood by those ofordinary skill in the art. Alternatively, LLR soft-decision outputvalues 211 may be combined by averaging to give a combined LLR value 213and the value may be passed to decoder 214, which may be designed towork with a correspondingly appropriate range of values.

EXAMPLE

FIG. 4 represents the performance advantages of joint-LLR diversitycombining according to one embodiment for an example with two receivesites. It is assumed that the sites are separated by a large enoughdistance to ensure decorrelation of atmospheric phenomena. Aside fromquantization errors in the soft-decision computation output (which canbe made negligible by proper choice of number of quantization levels),joint-soft-decision combining according to one embodiment may haveidentical performance to the coherent combining method of diversitycombining (but without the disadvantages of that method, as describedabove). In this plot, the overall link, e.g., combined link 109 providedby downlink beam 108, can be closed whenever the combined SNR operatingpoints of site1 (abscissa) and site2 (ordinate) lie to the right andabove the curves shown (curve 510 for joint-LLR or coherent combining;curve 520 for BER based switching combining). Thus, the simpler BERbased switching approach (curve 520) closes the link whenever one or theother site experiences SNR above the un-combined link SNR requirement,i.e., point 501 for site 1, point 502 for site 2. For example, anycombined operating point either to the right of, e.g. point 521, orabove, e.g., point 522, curve 520 experiences SNR above the un-combinedlink SNR requirement and closes the link using BER based switchingcombining. Coherent combining or joint-LLR combining according to oneembodiment (curve 510) permits even better performance: for example, inthe case of equal SNR into both sites, the coherent/LLR combining closesthe link for 3 dB lower SNR. Point 511, for example, experiences lowerSNR for both sites 1 and 2 than either of point 521 or point 522 yetcloses the link. In general, any operating point between curves 510 and520 closes the link using joint-LLR combining but not using BER basedswitching combining.

FIG. 5 illustrates method 600, in accordance with one embodiment of thepresent invention, for site diversity combining for achieving high linkavailability on a satellite or other radio frequency link that ischallenged by thin power margins and high probability of deep rain fade.At step 602 a signal—such as signal 103 or signal 205—may betransmitted, for example, from a satellite 102 using a modulated RFcarrier over a single downlink beam 108 which contains multiple receivesites—such as receive sites 104 a, 104 b—within its footprint 114.—Forexample, links 109 a, 109 b may provide a combined link 109 within thefootprint 114 of downlink beam 108—to a multiple number of receivesites—such as receive sites 104 a, 104 b—located at ground stations 104that may be separated by enough distance, nominally about 10 km, todecorrelate weather phenomena between the receive sites.

At step 604, the receive sites may be connected via a ground link—suchas a ground link 106 between receive sites 104 a, 104 b—to a centralreceive site, e.g., receive site 104 b in the exemplary embodimentillustrated in FIGS. 2A and 2B or intermediate site 105 in the exemplaryembodiment illustrated in FIG. 2C, for collecting data sent from thevarious receive sites. The connection over ground link 106 may be a highspeed digital connection, for example, or any link capable of providingthe synchronization needed for coherent or joint LLR combining. The datamay be in the form of, for example, IF band data 207, sampled values 209of a coded stream of symbols, or soft-decision (e.g., LLR) values ofsoft-decision output values 211.

At step 606, each copy of the signal received over the multiple linksmay be translated into soft-decision values (e.g., LLR values). Thetranslation may occur at each receive site separately with all thesoft-decision values collected at the central site, or alternatively,the necessary data for translation—such as soft-decision informationfrom each symbol (e.g. sampled values 209)—may be collected at thecentral site from each of the receive sites and translation for allcopies of the signal may be performed at the central site. At step 608,the soft-decision values—such as LLR soft-decision output values 211—maybe combined at one of the receive sites, e.g., the central site 104 b orintermediate site 105, for each corresponding symbol, i.e., copy, of theoriginal signal, e.g. signal 103 or signal 205. At step 610, theoriginal data of the signal may be decoded from the signal using thecombined soft-decision values—such as combined soft-decision values213—and turbo product decoder or suitable decoder for the originalencoding of the data, e.g., decoder 214.

It should be understood, of course, that the foregoing relates toexemplary embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

1. A method for site diversity combining, comprising the steps of:receiving a transmitted signal at at least two receive sites, thetransmitted signal includes coded symbols; translating, at a firstreceive site of the at least two receive sites, sampled symbol valuesfrom the coded symbols of the transmitted signal into a stream of firstsoft-decision values from the transmitted signal, the firstsoft-decision values being first log likelihood ratio values;translating, at at least one other receive site of the at least tworeceive sites, sampled symbol values from the coded symbols of thetransmitted signal into a stream of second soft-decision values from thetransmitted signal, wherein the second soft-decision values are secondlog likelihood ratio values; connecting to the at least one otherreceive site of the at least two receive sites that provides data forthe stream of second soft-decision values from the transmitted signal;digitally synchronizing the stream of first soft-decision values and thestream of second soft-decision values; and combining the firstsoft-decision values and the second soft-decision values into combinedsoft-decision values for each coded symbol of the transmitted signal. 2.The method of claim 1, further comprising the step of: separating thereceive sites far enough to decorrelate weather phenomena between thereceive sites.
 3. The method of claim 1, further comprising the step of:connecting the receive sites via a ground link.
 4. The method of claim1, further comprising the step of: sending the sampled symbol values ofcoded symbols via a ground link between the receive sites.
 5. The methodof claim 1, further comprising the step of: sending the soft-decisionvalues via a ground link between the receive sites.
 6. The method ofclaim 1, further comprising the step of: transmitting the signal usingbandwidth efficient modulation.
 7. The method of claim 1, wherein thecombining step averages the first log likelihood ratio values for acoded symbol of the transmitted signal with the second log likelihoodratio values for the coded symbol of the transmitted signal to providethe combined soft-decision values for the each coded symbol of thetransmitted signal.
 8. The method of claim 7, further comprising thestep of: decoding data from the transmitted signal using the combinedsoft-decision values for the each coded symbol of the transmitted tosignal to produce a set of decoded bit values from the coded symbol ofthe transmitted signal, the combined soft-decision values being averagedcombined log likelihood ratio values.
 9. The method of claim 1, whereinthe data for the stream of second soft-decision values include thesecond soft-decision values.
 10. A method for site diversity combining,comprising the steps of: receiving, at a first receiver located at afirst receive site, a modulated carrier signal transmitting a codedsymbol; receiving, at a second receiver located at a second receivesite, the modulated carrier signal transmitting the coded symbol,wherein at least one of the first and second receive sites is connectedvia at least one ground link; producing, using a first demodulator, afirst sampled symbol value from the coded symbol; producing, using asecond demodulator, a second sampled symbol value from the coded symbol;translating, using a first soft-decision computation module, the firstsampled symbol value into a first set of soft-decision values for thecoded symbol; translating, using a second soft-decision computationmodule, the second sampled symbol value into a second set ofsoft-decision values for the coded symbol that are digitallysynchronized with the first set of soft-decision values, wherein thefirst soft-decision computation module and the second soft-decisioncomputation module are at a same receive site and sampled symbol valuesare sent between the first and second receive sites via the at least oneground link; and combining, using a soft-decision combining module, thefirst set of soft-decision values and the second set of soft-decisionvalues to produce a set of combined soft-decision values for the codedsymbol, wherein data is sent from at least one of the first and secondreceive sites to the soft-decision combining module via the ground linkso that the first set of soft-decision values and the second set ofsoft-decision values are provided to the soft-decision combining module.11. A method of claim 10, further comprising: producing a set of decodedbit values for the coded symbol using a decoder connected to thesoft-decision combining module.
 12. A method of claim 10, wherein thefirst sampled symbol value is a pair I,Q of values, wherein I is anin-phase value and Q is a quadrature value determining the location ofthe coded symbol in a phase plane.
 13. A method of claim 10, wherein thefirst soft-decision computation module translates an I,Q pair of thefirst sampled symbol value from the coded symbol into the first set ofsoft-decision values that are n-bit representations of the likelihood ofeach bit of a bit mapping for a modulation scheme of the modulatedcarrier signal transmitting the coded symbol.
 14. A method for sitediversity combining, comprising: transmitting, using a transmitter thatis located on a satellite, a coded symbol via a modulated carriersignal, using a bit mapping of coded symbols to a modulation scheme,over a combined link comprising at least two links, including a firstlink and a second link; connecting, via the first link, a first receivesite to the transmitter, the first receive site including a firstreceiver that receives the modulated carrier signal and a firstdemodulator that demodulates the modulated carrier signal and produces afirst sampled symbol value I,Q pair from the coded symbol; connecting,via the second link, a second receive site to the transmitter, thesecond receive site including a second receiver that receives themodulated carrier signal and a second demodulator that demodulates themodulated carrier signal and produces a second sampled symbol value I,Qpair from the coded symbol, wherein the first receive site and thesecond receive site are separated from each other by a distance thatallows decorrelation of weather phenomena; the first receive site andthe second receive site being connected via a ground link to provide thefirst sampled symbol value I,Q pair and the second sampled symbol valueI,Q pair to a joint soft-decision computation module to compute acombined soft-decision output value for the coded symbol; and receiving,by a decoder that is connected to the joint soft-decision computationmodule, the combined soft-decision output value and producing a set ofdecoded bit values for the coded symbol.
 15. The method of claim 14,wherein the transmitter uses bandwidth efficient modulation; and themodulation scheme is chosen from quadrature amplitude modulation,quadrature phase shift keying, binary phase shift keying, and amplitudephase keying.